The present invention relates to what is commonly designated as a five-speed automatic transmission that is particularly adapted for vehicular usage. That is, the transmission provides five forward speeds or gear ratios, and one reverse speed or gear ratio. By way of introduction, and to clarify the discussion which follows, the forward gear ratios are defined as those ratios which cause the vehicle to move forwardly and for simplicity, it will be assumed that the configuration of the planetary gear sets is such that in a forward gear ratio the output member rotates in the same direction as the input member. Conversely, the reverse ratio causes the vehicle to move rearwardly, and it will also be assumed that the output member rotates in a direction opposite to that of the input member.
As is also known, an input member is the connecting mechanism (normally a shaft in the transmission) which receives its driving force or torque from the vehicular engine--as through a well known torque converter--and imparts the driving torque to the planetary gear sets which comprise the epicyclic transmission. The also well known output member is that mechanism (normally also a shaft) which operatively connects the transmission to the differential from which the drive wheels are rotated.
"Torque transfer device" is another term commonly employed in the description of planetary gear sets. Two forms of torque transfer devices are commonly recognized--viz.: input clutch members and reaction brake members. An input clutch member is employed selectively to interconnect two members that are normally relatively rotatable so that they move in unison. A reaction brake member is employed selectively to preclude rotation of a member that is mounted so as be rotatable. Typically, a reaction brake member is presented from the housing in which the transmission is encased, and the reaction brake member thereby serves to "ground" a normally rotatable member to the housing so that member cannot rotate.
Most automatic transmissions known to the prior art would have required more than two planetary gear sets to achieve the five forward speeds or gear ratios provided by the present transmission.
A planetary gear set typically consists of a sun gear, which is a small gear located in the center of the planetary set, and a ring gear which, as the outermost member, circumscribes the set and has inwardly facing teeth. A plurality of planetary gears or pinions are interposed between the aforementioned sun and ring gears to be rotatable and/or circumferentially translatable. The pinions in each planetary gear set are normally supported from a carrier so that as the pinions translate the carrier will rotate in response to translation of the pinions. Conversely, the carrier may be rotated to effect a desired movement of the pinions. Rotation of the sun gear will cause all of the other elements in the planetary set to move, as well, unless one of the other of said elements is held stationary by a torque transfer device, such as a reaction brake member, which may be presented from the transmission housing. When a reaction brake member grounds one of the components in a planetary gear set to the housing, the grounded member will be forcibly held in position so that all the other members are movable relative thereto. Selectively grounding the members of a planetary gear set in combination with selectively connecting the input shaft to the desired component of the planetary gear set constitutes the typical means by which different gear ratios are achieved from a planetary gear set--i.e.: different members are selected to be rotated or grounded s the speed and direction of the output can be appropriately changed to accommodate the existing conditions.
There is, however, a limit to the number of variations which can be provided by a single planetary set. As a result, planetary gear sets have been compounded to provide a greater number of individual speeds or gear ratios. When planetary gear sets are compounded, various torque transfer devices are provided to effect selective interconnection between the members of the individual gear sets, as well as to ground one or more of the components of the gear sets to the transmission housing. These arrangements, while successful, do tend to increase the length of the housing required to encase not only the gear sets but also the torque transfer devices required to provide the desired number of gear ratios.
One rather unique arrangement for effecting the desired compounding for a pair of planetary gear sets was disclosed in U.S. Pat. No. 3,763,719 issued Oct. 9, 1973, to Malloy, and assigned to the assignee of the present invention. In that arrangement a pair of planetary gear sets was compounded by employing a unique commonality of the pinions. The arrangements disclosed in the aforesaid patent provides four forward gear ratios by the selective actuation of five torque transfer devices. That arrangement does not, however, provide five forward gear ratios.